1. Field of the Invention
The present invention relates to a coating composition which is used to form an inorganic film on a substrate made of a material such as glass, ceramics, metals or plastics, and a process for manufacturing the same.
2. Description of the Prior Art
Inorganic films are formed for a variety of purposes on the surfaces of substrates made of materials such as glass, ceramics, metals and plastics. The film imparts specific electrical, optical, chemical or mechanical properties to the substrate material. Specific purposes of the films include electrical conduction or insulation, the selective transmission or absorption of light, the prevention of elution of an alkaline substance, and the formation of a film having chemical resistance, or a hard coating film.
These films are formed by a gas-phase process, such as CVD, PVD or sputtering process, or by a liquid-phase process employing an alkoxide compound, etc. The gas-phase process usually requires an expensive apparatus, such as one for vacuum deposition. The size and shape of the substrate on which a film can be formed are limited by the size of the apparatus which is employed.
The sol-gel process employing an alkoxide compound, etc. has the advantage of being capable of forming a film on a surface having a large area. When this process is employed, however, it is often difficult to maintain a coating solution of good quality and achieve a satisfactory control of the atmosphere, including the control of humidity, as all the alkoxide compounds, except silicon alkoxide, are rapidly hydrolyzed (Japanese Patent Laid-open Publication No. Sho 52-138512 (138512/1977)).
An attempt has been made to prepare a coating solution by adding a chelating agent, such as acetylacetone, to control the rate at which an alkoxide compound is hydrolyzed (Japanese Patent Laid-open Publication No. Sho 63-258959 (258959/1988)). However, the chelated compound usually has a higher thermal decomposition temperature and need be heated to a temperature of at least 450 .degree. C.
An attempt has also been made to prepare a transparent coating solution by adding a mineral acid to the hydrolyzed product of silicon alkoxide and titanium alkoxide, while not relying upon any stabilizing method, such as chelating (Japanese Patent Laid-open Publication No. Sho 55-25487 (25487/1980)). This is known as a silica-titania coating solution. This solution is, however, low in storage stability, particularly to water.
There is known a coating solution using silicon alkoxide, which is hydrolyzable only slowly (Japanese Patent Publication No. Sho 63-58867 (58867/1988)). This solution is used to form, for example, a film for preventing the elution of an alkali. This solution, however, can only form a film of low mechanical strength when baked at a lower temperature.
There is also known a silica-alumina coating solution. There has been reported an ethanol solution of tetraethoxysilane and aluminum nitrate as a mullite coating solution Speech #2G39 at the Annual Meeting of the Ceramic Society of Japan in 1987). This solution is, however, low in film-forming stability. The mullite coating solution can form a uniform film if used immediately after its preparation, but if used after several days of storage, the separation of its aluminum component occurs during its application or drying.